1 Field of the Invention
This invention relates to a method for operating a light-radiant heating furnace which uses incandescent lamps as its light-radiant source.
2 Description of the Prior Art
Among a variety of apparatus adapted to carry out heat treatments therein, light-radiant heating furnaces in which light radiated from an incandescent lamp or lamps is irradiated onto objects or materials to be treated (hereinafter referred to merely as "objects") for their heat treatment have the following merits:
(1) Owing to an extremely small heat capacity of an incandescent lamp per se, it is possible to raise or lower the heating temperature promptly;
(2) The heating temperature can be easily controlled by controlling the electric power to be fed to the incandescent lamp;
(3) Since they feature indirect heating by virtue of light radiated from their incandescent lamps which are not brought in contact with the objects, there is little danger of contaminating objects under heat treatment;
(4) They enjoy less energy consumption because full-radiation-state operation of the lamps in feasible a short time after turning the lamps on and the energy efficiencies of the lamps are high; and
(5) They are relatively small in size and inexpensive compared with conventional resistive furnaces and high-frequency heating furnaces.
Such light-radiant heating furnaces have been used for the heat treatment and drying of steel materials and the like and the molding of plastics as well as in thermal characteristics testing apparatus and the like. Use of light-radiant heating furnaces has particularly recently, been contemplated to replace the conventionally-employed resistive furnaces and high-frequency heating furnaces for carrying out certain semiconductor fabrication processes which require heating, for example, diffusion processes of dopant atoms, chemical vapor deposition processes, annealing processes for healing crystal defects in ionimplanted layers, thermal treatment processes for activation, and thermal processes for nitrifying or oxidizing the surfaces of silicon wafers. As reasons for the above move may be mentioned the incapability of conventional heating furnaces to heat heating larger-sized objects uniformly, thereby failing to meet the recent trend toward larger semiconductor wafer size, in addition to the advantages of light-radiant heating furnaces that objects under heat treatment are free from contamination, their electric properties are not deleteriously affected and the light-radiant heating furnaces require less power consumption.
In such a light-radiant heating furnace, there is provided a suitable transportation system for transporting objects intermittently. By the transportation system, each object is transported into the light-radiant heating furnace and then subjected to a heat treatment therein. Namely, each object is held on a carrier of the transportation system at the loading station for the objects. Thereafter, the carrier is caused to move into the light-radiant heating furnace in which the object held on the carrier is exposed to light radiated from incandescent lamps, which make up a light-radiant source, so as to carry out the heat treatment. Then, the carrier is again caused to move to the unloading station of treated objects, which unloading station is located outside the light-radiant heating furnace. At the unloading station, the thus-treated object is unloaded from the carrier, thereby completing a single cycle of the heat treatment process. The carrier is thereafter moved again to the loading station and loaded with the next object to be treated. Then, the heat treatment process of the next object is carried out in the same manner.
Heat treatment of objects is generally carried out in such a manner as mentioned above. Depending on the kinds or types of objects to be treated, their heating temperatures and heating time periods must be strictly controlled.
For example, the ion implantation process has recently been finding actual utility as an effective method for introducing dopant atoms into a semiconductor wafer (hereinafter referred to as "wafer") since it is possible to control accurately the concentration levels of dopant atoms and the depths of resulting junctions. In the ion implantation process, it is necessary, subsequent to implantation, to subject each wafer to a heat treatment at about 1000.degree. C. or higher. This heat treatment must be carried out accurately in a short time period so as to prevent the concentration distribution of the implanted dopant atoms in the depthwise direction of the wafer from changing due to thermal diffusion. Furthermore, there is an outstanding demand for the establishment of a high-speed heating and cooling cycle for wafers in order to improve their productivity.
It has however been found that, when a wafer, for example, a wafer of a single crystal of silicon is heated to 1000.degree. C. or higher in a few seconds, a damage called "slip line" is developed in the wafer because of a difference in the rising velocity of temperature between its peripheral portion and its central portion, in other words, due to a non-uniform temperature rise therebetween. The thickness of a wafer is generally very small, namely, of a level of about 0.5 mm or so and its thicknesswise temperature distribution is thus rendered substantially uniform in a very short time period of a level of 10.sup.-3 second or so. Accordingly, such a damage as slip line may be possibly avoided if the temperature distribution on the surface of each wafer is rendered uniform. However, it is very difficult to prevent the development of such a damage as slip line in an actual process even if the surface of each wafer is exposed to radiated light having a uniform irradiation energy density, because more heat radiates from the peripheral portion of the wafer than its central portion and the peripheral portion thus remains cooler than the central portion
Furthermore, in repeated heating processes of sensitive objects such as silicon wafers, it is necessary to subject each object to a heat treatment under the same conditions. If each object is subjected to the treatment under different conditions, the treated objects would have non-uniform characteristics due to the differences of the heating conditions, even when the differences are small.